D

Intermediate mesoderm

Neuromere And Somitomere Development

Figure 5.9 Transverse sections showing development of the mesodermal germ layer. A. Day 17. B. Day 19. C. Day 20. D. Day 21. The thin mesodermal sheet gives rise to paraxial mesoderm (future somites), intermediate mesoderm (future excretory units), and lateral plate, which is split into parietal and visceral mesoderm layers lining the intraembryonic cavity.

Electron Microscopy Paraxial Mesoderm

Figure 5.10 Transverse sections through cervical somites of mouse embryos (approximately 21-day human) as visualized by scanning electron microscopy. Arrow, noto-chord; arrowhead, neural canal; En, endoderm; Ic, intraembryonic cavity; Ne, neuroec-toderm; S, Somite; So, somatic mesoderm; and Sp, splanchnic mesoderm.

Figure 5.10 Transverse sections through cervical somites of mouse embryos (approximately 21-day human) as visualized by scanning electron microscopy. Arrow, noto-chord; arrowhead, neural canal; En, endoderm; Ic, intraembryonic cavity; Ne, neuroec-toderm; S, Somite; So, somatic mesoderm; and Sp, splanchnic mesoderm.

segmentation of the neural plate into neuromeres and contribute to mesenchyme in the head (see Chapter 15). From the occipital region caudally, somitomeres further organize into somites. The first pair of somites arises in the occipital region of the embryo at approximately the 20th day of development. From here, new somites appear in craniocaudal sequence at a rate of approximately three pairs per day until, at the end of the fifth week, 42 to 44 pairs are present (Figs. 5.3, 5.5, and 5.8). There are four occipital, eight cervical, 12 thoracic, five lumbar, five sacral, and eight to 10 coccygeal pairs. The first occipital and the last five to seven coccygeal somites later disappear, while the remaining somites form the axial skeleton (see Chapter 8). During this period of development, the age of the embryo is expressed in number of somites. Table 5.2 shows the approximate age of the embryo correlated to the number of somites.

By the beginning of the fourth week, cells forming the ventral and medial walls of the somite lose their compact organization, become polymorphous, and shift their position to surround the notochord (Fig. 5.11, A and B). These cells, collectively known as the sclerotome, form a loosely woven tissue, the mesenchyme. They will surround the spinal cord and notochord to form the vertebral column (see Chapter 8). Cells at the dorsolateral portion of the somite also migrate as precursors of the limb and body wall musculature (Fig. 5.11 B). After migration of these muscle cells and cells of the sclerotome, cells at the dorsomedial portion of the somite proliferate and migrate down the ventral side of the remaining dorsal epithelium of the somite to form a new layer, the myotome (Fig. 5.11, B and C). The remaining dorsal epithelium forms the dermatome, and together these layers constitute the dermomyotome (Fig. 5.11 C). Each segmentally arranged myotome contributes to muscles of the back (epaxial musculature; see Chapter 9), while dermatomes disperse to form the dermis and subcutaneous tissue of the skin (see Chapter 18). Furthermore, each myotome and dermatome retains its innervation from its segment of origin, no matter where the cells migrate. Hence each somite forms its own table 5.2 Number of Somites Correlated to Approximate Age in Days

Approximate Age (days)

No. of Somites

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